The events which link the action potential and the subsequent contraction (excitation contraction coupling) in skeletal muscle have been ascribed a sequence involving the transmission of the electrical signal into the cell via the T System and subsequent release and sequestration of Ca by components of the longitudinal sarcoplasmic reticulum (L.S.R.). This sequence, though fairly well accepted, is to no small degree conjectural and is based on the piecing together of many lines of evidence. A study of Ca flux from single skeletal muscle fibers has shown that changes in intracellulr Calcium concentration are generally reflected by changes in Ca flux. The efflux kinetics suggest that the main Ca store (LSR) is of major importance in regulating contraction and relaxation. This Ca store is difficult to study in intact living skeletal muscle fibers because of the anatomical position of the LSR. Flux measurement from this compartment are complicated by the presence of the T system. In the proposed research we would approach this problem by exploiting a muscle, the anterior byssal retractor of Mytilus edulis, which apparently lacks a T system and has its LSR lined up in close contact with the cell mebrane. The proposed studies are expected to yield information on (1) Ca flux in relation t activation and the mechanisms by which the LSR mmbranes control Ca flux; (2) The effect of transmitters such as serotonin, catecholamines, and acetylcholine on Ca flux and the mechanisms by which they change Ca flux; (3) The identity and localization to their activity; (4) Basic mechanisms by which membranes control Ca levels.